1. Field of the Disclosure
The present invention relates generally to electrophotographic image forming devices and more particularly to a charge roll mounting assembly for an electrophotographic image forming device.
2. Description of the Related Art
As is well known in the art, during a print operation by an electrophotographic image forming device a charge roll charges the surface of a photoconductive drum to a predetermined voltage. The charged surface of the photoconductive drum is then selectively exposed to a laser light source to selectively discharge the surface of the photoconductive drum and form an electrostatic latent image on the photoconductive drum corresponding to the image being printed. Toner is picked up by the latent image on the photoconductive drum from a developer roll (in the case of single component development printing) or a magnetic roll (in the case of dual component development printing) creating a toned image on the surface of the photoconductive drum. The toned image is then transferred from the photoconductive drum to the print media either directly by the photoconductive drum or indirectly by an intermediate transfer member. A cleaning blade or roller removes any residual toner adhering to the photoconductive drum after the toner is transferred from the photoconductive drum. The cleaned surface of the photoconductive drum is then ready to be charged again and exposed to the laser light source to continue the printing cycle.
The charge roll is preferably biased uniformly along the axial length of the charge roll against the surface of the photoconductive drum to provide uniform charging across the axial length of the photoconductive drum. If the charge roll bias is uneven, print defects may occur. For example, if the charge roll does not make enough contact at the axial ends of the photoconductive drum for proper charging, dark spots will occur at the edges of the printed page. On the other hand, if the charge roll has too much bias at the axial ends of the photoconductive drum, light or feathered printing will occur at the edges of the printed page. The charge roll is often driven by friction from the nip formed between the charge roll and the photoconductive drum. If the nip force is too low, the charge roll may slip against the surface of the photoconductive drum resulting in dark bands on the printed page. During long periods of inactivity, such as during shipping or storage of the image forming device or a replaceable unit containing the charge roll and photoconductive drum, a flat spot may be formed on the surface of the charge roll where it contacts the photoconductive drum due to compression of the charge roll at that location. When printing resumes, the flat spot causes a temporary spike in the load to the charge roll that can't be overcome if the nip force is too low until after the image forming device operates long enough for the charge roll to regain its original shape. Further, excessive vibration of the charge roll during operation may cause light or dark bands on the printed page as a result of the charge roll momentarily having a bias that is too high or too low as it vibrates.
FIGS. 1 and 2 show a prior art charge roll mounting assembly 20. Assembly 20 includes a cleaner bracket 22 having a rear plate 24 and a top plate 26 that each extend in a lengthwise direction 28 corresponding with the axial direction of the photoconductive drum (FIG. 3). Top plate 26 extends forward and upward from rear plate 24. Rear plate 24 and top plate 26 are formed integrally with each other from electrogalvanized steel sheet. A cleaner blade 30 extends in a cantilevered manner downward from rear plate 24. A free end 32 of cleaner blade 30 is positioned to contact the surface of the photoconductive drum to remove residual toner from the photoconductive drum.
A charge roll 34 is mounted to cleaner bracket 22 in position to contact the surface of the photoconductive drum to charge the surface of the photoconductive drum. A cleaner roll 36 is mounted against charge roll 34 to clean toner from the surface of charge roll 34. Charge roll 34 includes a shaft 35 and cleaner roll 36 includes a shaft 37. Cleaner roll 36 is driven by friction from the nip formed between charge roll 34 and cleaner roll 36. The axial ends of shafts 35 and 37 are retained by bearings 38A, 38B. Specifically, each bearing 38A, 38B includes a charge roll opening 40A, 40B that receives an axial end of shaft 35 and a cleaner roll opening 42A, 42B that receives an axial end of shaft 37. Openings 40A, 40B, 42A, 42B are generally cylindrical in shape and formed by bearing surfaces for shafts 35 and 37 of charge roll 34 and cleaner roll 36 to rotate against. The distance between openings 40A and 42A and between openings 40B and 42B define the positional relationship between charge roll 34 and cleaner roll 36 to achieve the desired nip force between charge roll 34 and cleaner roll 36. Cleaner roll openings 42A, 42B are spaced axially inward from charge roll openings 40A, 40B due to the shaft of cleaner roll 36 having a shorter length than the shaft of charge roll 34.
A cast zinc bearing retainer 44A, 44B mounts each bearing 38A, 38B to cleaner bracket 22 on inner axial sides of bearing retainers 44A, 44B. Each bearing retainer 44A, 44B includes a rectangular slot 46A, 46B that slips over a corresponding flange 48A, 48B formed at each end of top plate 26 to align bearing retainers 44A, 44B with cleaner bracket 22. Bearing retainers 44A, 44B and rear plate 24 of cleaner bracket 22 have corresponding screw holes 50A, 50B and 52A, 52B that receive a screw at each end of cleaner bracket 22 to fix bearing retainers 44A, 44B to cleaner bracket 22 and cleaner bracket 22 to a housing of the image forming device or a housing of a replaceable unit of the image forming device. Bearing retainer 44B includes a fixed pin 56 that extends axially inward that retains bearing 38B on bearing retainer 44B. Bearing retainer 44A includes a guide slot 58 in substantially the same position on bearing retainer 44A as pin 56 on bearing retainer 44B. Guide slot 58 receives a locking pin 60 that retains bearing 38A on bearing retainer 44A as discussed in greater detail below.
FIG. 3 shows an end view of bearing 38A positioned relative to cleaner bracket 22 with bearing retainer 44A removed to more clearly illustrate the features of bearing 38A. Bearing 38B is substantially the same as bearing 38A except that bearing 38B is a mirror image of bearing 38A. Each bearing 38A, 38B includes an arm 62 that extends forward, away from rear plate 24, from the portion of the bearing 38A, 38B that forms charge roll opening 40A, 40B. An opening 64 is formed in a distal end of arm 62. Opening 64 of bearing 38B receives pin 56 of bearing retainer 44B and opening 64 of bearing 38A receives locking pin 60. Each bearing 38A, 38B is pivotally mounted to its bearing retainer 44A, 44B and cleaner bracket 22 about a pivot point 66 at the center of opening 64. A compression spring 68A, 68B is positioned between each flange 48A, 48B of top plate 26 and a ledge 70 formed on a top surface of arm 62. Each ledge 70 includes a small finger 72 extending from ledge 70 that fits inside the end of compression spring 68A, 68B that is positioned against ledge 70 to position the end of compression spring 68A, 68B nearest ledge 70. A spring screw 74A, 74B passes through a screw hole 76A, 76B in each flange 48A, 48B and into the end of compression spring 68A, 68B that is positioned against flange 48A, 48B to position the end of compression spring 68A, 68B nearest flange 48A, 48B. Compression springs 68A, 68B bias bearings 38A, 38B about pivot point 66 toward a photoconductive drum 33 (in a counterclockwise direction as viewed in FIG. 3). Charge roll 34 and cleaner roll 36 move about pivot point 66 as a result of their engagement with charge roll openings 40A, 40B and cleaner roll openings 42A, 42B of bearings 38A, 38B. In this manner, the force from compression springs 68A, 68B biases charge roll 34 against photoconductive drum 33.
When charge roll mounting assembly 20 is installed in the image forming device and positioned relative to photoconductive drum 33, photoconductive drum 33 applies a force on charge roll 34 in the direction of the arrow 78 shown in FIG. 3. The force from photoconductive drum 33 on charge roll 34 compresses compression springs 68A, 68B from their home positions causing bearings 38A, 38B to pivot away from photoconductive drum 33 (in the clockwise direction as viewed in FIG. 3), in turn, displacing charge roll 34 from its home position to a position biased against the outer surface of photoconductive drum 33.
FIG. 4 shows locking pin 60 in greater detail. Locking pin 60 includes a handle 80 that includes a relatively wide base 82 and a narrower flange 84 that extends from base 82. A rectangular prism shaped rod 86 extends away from base 82 of handle 80 in a direction generally orthogonal to handle 80. The rectangular cross section of rod 86 is defined by a height and a width. The height is too large to fit through a channel 59 at the front of bearing retainer 44A that forms an entrance to guide slot 58 but the width is small enough to pass through channel 59 in order to permit locking pin 60 to be removed from bearing retainer 44A as discussed below. Rod 86 leads to a cylindrical spacer 88. A cylindrical pin 90 extends from spacer 88 away from handle 80. Spacer 88 is concentric with rod 86 and pin 90 and has a diameter that is larger than the height and width of rod 86 and the diameter of pin 90. The portion of spacer 88 that extends radially beyond the outer surface of rod 86 is spaced from handle 80 by the length of rod 86 in the axial direction of charge roll 34. A retaining bump 92 extends slightly outward from flange 84 in the same direction as rod 86 and pin 90. With reference to FIGS. 1-4, pin 90 is positioned in opening 64 on arm 62 of bearing 38A. The engagement between pin 90 and opening 64 of bearing 38A controls the position of pivot point 66 of bearing 38A relative to bearing retainer 44A. Rod 86 is positioned in guide slot 58 of bearing retainer 44A with bearing retainer 44A sandwiched between spacer 88 and base 82 of handle 80 to position locking pin 60 axially relative to bearing retainer 44A. Retaining bump 92 extends into a corresponding opening 94 in bearing retainer 44A that is positioned above guide slot 58. The engagement between positioning bump 92 and opening 94 prevents locking pin 60 from rotating relative to bearing retainer 44A. When positioning bump 92 is positioned in opening 94, rod 86 is oriented with its height aligned with channel 59 such that rod 86 cannot slide out of guide slot 58 and locking pin 60 cannot separate from bearing retainer 44A.
Locking pin 60 is manually installable onto and removable from charge roll mounting assembly 20 to aid in the installation and removal of charge roll 34 and cleaner roll 36 onto and off of cleaner bracket 22. To remove locking pin 60, a user pulls flange 84 of handle 80 away from bearing retainer 44A until positioning bump 92 pulls out of opening 94. Locking pin 60 is then free to rotate relative to bearing retainer 44A until the width of rod 86 is aligned with channel 59 so that rod 86 can slide out of guide slot 58 and locking pin 60 can separate from bearing retainer 44A. Pin 90 can then be removed from opening 64 on arm 62 of bearing 38A. To reengage locking pin 60 with bearing 38A, this sequence is reversed.